Japanese translation of PCT publication No. H11-500828 introduces a detector for detecting a rotation angle of a main rotator, such as a steering wheel of an automobile that rotates beyond 360° within a predetermined rotation range. The conventional detector contains a first gear fitted with the main rotator and at least two rotators, i.e., a second rotator and a third rotator that cooperate with the first gear. The second and the third rotators have a second and a third gears, respectively. Each sensor for the second and the third rotators detects each rotation angle of the two rotators, and from the phase difference between the second and the third rotators, the rotation angle of the main rotator is found.
According to the conventional rotation angle-detecting device, however, the first gear through which a steering shaft is fitted has a large diameter, accordingly, the diameter of a bearing inevitably becomes large. Therefore, the first gear tends to have a distortion. To avoid the inconvenience, the first gear has to be loosely fitted with the steering shaft. Similarly, a large clearance is required in each engagement between the first and the second gears, and between the second and the third gears. The loose fit brings a relatively large play between the second and the third gears, and therefore, an error of measurement occurs in the two sensors in evaluating an absolute angle.
As another inconvenience in the conventional structure, in reducing the size of the device, the magnets built in the second and the third gears interfere magnetic lines with each other, thereby degrading the accuracy in determining the rotation angle.